1. Field of the Invention
The present application discloses a method for reducing blade vibration in turbo-machinery. The method involves inserting flow obstructions and/or gas injections upstream of vibrating blades in such a manner that power flow into the blades is reduced by means of cancellation within a modal power integral.
2. Description of the Related Art
Rotor blades in turbo-machinery are excited into resonant vibrations by inhomogeneities in the flow stream. These excitations cause cyclic stress, resulting in high cycle fatigue and premature failure in the blades. The blades are excited to large amplitude when a blade modal frequency corresponds to the shaft rotational frequency multiplied by the harmonic number of the flow inhomogeneity seen by the blade. Typically the number of resonances with amplitude large enough to cause high cycle fatigue is limited. Since the damage rate from fatigue is approximately proportional to the sixth power of the cyclic stress amplitude, a modest reduction in the vibration amplitude often will eliminate high cycle fatigue as the limiting factor for blade life.
One current practice is to avoid operation at the resonant frequency by changing the speed rapidly when a resonance is encountered, thereby minimizing the number of fatigue cycles that a blade accumulates. If the number of vibration cycles is kept small by this strategy, then blade failure will be controlled by other mechanisms. However, this practice places undesirable limits on the operation of turbo-machinery and the aircraft.
Another current approach is to reduce the spatial variations in the flow field by directly injecting air into low-velocity wakes behind obstructions (Rao, N. M., Feng, J., Burdisso, R. A, and Ng, W. F., xe2x80x9cActive Flow Control to Reduce Fan Blade Vibration and Noisexe2x80x9d, 5th AIAA/CEAS Aeroacoustic Conference, American Institute of Aeronautics and Astronautics, May 10-12, 1999). This approach requires the use of either air from the compressor or from an additional external air source in relatively large quantities. Use of compressor air has a detrimental impact on performance. The addition of a separate air supply adds weight and requires power. Both methods have detrimental impacts on performance. Also, wake filling does not address modal excitation due to bow waves from down stream flow obstructions.
The current invention is a method of reducing blade excitation. The invention employs a control system and aerodynamic elements that modify the spatial distribution of flow striking the blades. The flow distribution is modified so as to reduce the power flowing into a mode by orthogonalizing the unsteady pressure field on the surface of the blade and the modal velocity distribution on the surface of the blade. One means to this end is to insert simple obstructions into the flow just upstream of the resonating rotor blades. These obstructions need only be deployed when the rotor speed corresponds to resonant excitation. They can be mounted on the case, vanes, or struts. Various control modes are possible. The control system can include: (1) a fixed position, (2) a very simple, open-loop, scheduled deployment, and/or (3) a feedback control system with sensors to measure blade vibration.
More specifically, the present invention is for a system for reducing blade vibration in turbo-machinery. The system comprises placing physical or mechanical obstructions upstream of vibrating blades in turbo-machinery in a manner so that power flow into blade vibration is reduced by means of cancellation within a modal power integral, where the obstructions are selected from the group consisting of flow obstructions, gas injections and combinations of flow obstructions and gas injections.
In preferred embodiments of the system, the obstructions are located on an interior surface of a turbo-machine case to achieve such cancellation; the obstructions are located on or behind vanes of a turbo-machine to achieve such cancellation; or the obstructions are located on or behind struts of a turbo-machine to achieve such cancellation.
In more preferred embodiments of the system, reconfiguration of the flow obstructions is scheduled on the basis of rpm of the blades; reconfiguration of the flow obstructions is adjusted using blade vibration sensors; and/or the obstructions are in a fixed position.